Current production automobiles typically incorporate power steering systems wherein a driver's effort to steer are assisted by powered systems, such as a hydraulic systems that is driven electrically or mechanically from a pump attached to the engine. However, electrically-driven cars or hybrid cars typically do not have a running motor to drive a hydraulic system when the cars are not moving. In this case, there is no power-assist to help a driver turn the wheels of these cars, such as when trying to negotiate a parking spot. A hydraulic system could be driven by an electric motor, but this is inefficient as an electrical system could be used directly to provide steering assist. Further electrical power assist need not be available full time, as in a hydraulic system, but only as needed by a driver. As a result power is conserved and total fuel efficiency is increased.
In an Electric Power Steering (EPS) system, steering torque assist is provided directly by an electric motor drive. The motor itself can include a three-phase permanent magnet synchronous motor, as is known in the art. This motor is driven from the vehicle battery through application of an inverter system, as is also known in the art. The drive is typically monitored and controlled by a processing unit with sensor to detect the operating conditions of the motor. The EPS drive assist is called into use based on driver demand. Specifically, as a driver applies a steering force in one direction, the EPS system supplies a further torque in the same direction. However, in case of system failure, the torque from the EPS motor can generate unintended steering torque. For example, a driver could be turning left while the motor drive erroneously provides a torque to the right, which is undesirable.
One prior art technique to detect unintended steering torque is to compare a steering torque current to a calculated steady-state current. If the discrepancy between the currents is larger than a predetermined limit, for a predetermined time period, a fault is indicated. The problem with this technique is a relatively long time delay. In particular, since this technique is based on steady-state equations, a relatively large time delay must be allowed for settling to avoid false triggering. Moreover, if the motor is stalled, no current is measured and there is no fault detection.
Another prior art technique uses two parallel processors to receive sensor signals and to compute a motor control algorithm from these sensor inputs. In this case, the outputs from the two processors are compared and, if they do not match, a fault is indicated. This technique is limited to detecting faults in the processing system and not in the motor itself or the motor drive hardware (e.g. inverter). Moreover, the use of two processors adds significant cost.
Therefore, what is needed is an EPS controller that can detect unintended steering assist. It would also be an advantage to detect any unintended steering with a minimum time delay so that the motor output can be shut off before the system could degrade a driver's ability to steer. It would also be of benefit to provide this advantage in a cost effective manner.